Preparation of substantially dust-free tetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione granules

ABSTRACT

Substantially dust-free granules of tetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione (I) are prepared by reacting methylamine (II) with carbon disulfide (III) and formaldehyde (IV) or by reacting the methylammonium salt of N-methyldithiocarbamic acid (V) with formaldehyde (IV) by a process in which the reaction is carried out in the presence of at least one diaminoalkylene of the formula VI 
     
         R.sup.1 -NH-A-NH-R.sup.2                                   VI 
    
     where R 1  and R 2  independently of one another are each hydrogen or alkyl and A is an unsubstituted or substituted 1,2-ethylene, 1,3-propylene or 1,4-butylene bridge.

This application is a 371 of PCT/EP92/02907, filed Dec. 15, 1992. Thepresent invention relates to a process for the preparation ofsubstantially dust-free granules oftetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione (I) by reactingmethylamine (II) with carbon disulfide (III) and formaldehyde (IV) or byreacting the methylammonium salt of N-methyldithiocarbamic acid (V) withformaldehyde (IV).

The invention also relates to substantially dust-free granules oftetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione and to methods ofsoil decontamination and of controlling nematodes, germinating plantsand soil fungi using these granules.

Tetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione (I) (common name:dazomet) is used in agriculture and horticulture for soildecontamination (against nematodes, germinating plants and soil fungi)(U.S. Pat. No. 2,838,389).

In the known preparation processes, the active ingredient is obtained inthe form of a fine powder which furthermore has a high content of activeingredient as dust. Such a product is not suitable for the safe use ofthe active ingredient, which releases methyl isothiocyanate ondecomposition.

The literature discloses that thiadiazine derivatives, such astetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione (I), are obtained inthe form of granules if the reaction of the starting materials iscarried out in the presence of an emulsifier (Emulgen PP150) and zincsulfate (JP-A 84/210 073=Chemical Abstracts 102 (19), 166 783 g). Thegranules thus prepared contain 10% of particles having a diameter offrom 200 to 300 μm, 79% of particles having a diameter of from 100 to200 μm and 11% of particles having a diameter of less than 100 μm.

However, problems with the disposal of the aqueous mother liquors mayoccur in this process, owing to the use of the inorganic salt and of theemulsifier.

It is an object of the present invention to provide a simpler processfor the preparation oftetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione (I) in the form ofgranules.

We have found that this object is achieved by a process for thepreparation of substantially dust-free granules oftetrahydro-3,5-dimethyl1,3,5-thiadiazine-2-thione (I) by reactingmethylamine (II) with carbon disulfide (III) and formaldehyde (IV) or byreacting the methylammonium salt of N-methyldithiocarbamic acid (V) withformaldehyde (IV), wherein the reaction is carried out in the presenceof at least one diaminoalkylene of the formula VI

    R.sup.1 -NH-A-NH-R.sup.2                                   VI

where R¹ and R² independently of one another are each hydrogen or C₁ -C₄-alkyl and A is a 1,2-ethylene, 1,3-propylene or 1,4-butylene bridge andthese bridges may carry from one to four C₁ -C₄ -alkyl groups.

The reaction is carried out according to the following reaction scheme:##STR1##

The process may be based on the fact that small amounts of thediaminoalkylene compound compete with methylamine in the reaction and,if R¹ and R² are simultaneously hydrogen, for example "dimers" of theformula VII or higher "polymers" of the active ingredient may thus beformed. ##STR2##

The diaminoalkylene compounds in which R¹ or R² is not hydrogen myresult in the formation of, for example, byproducts of the followingstructure VIII. ##STR3##

Diaminoalkylene compounds in which neither R¹ nor R² is hydrogen mightreact with carbon disulfide to give noncyclized products of thestructure IX.

    R.sup.1 -N.sup.+ H.sub.2 -A-NR.sup.2 -CS.sub.2             IX

In addition to the possible byproducts described above, other structuresare also possible.

However, the fact that the possible byproducts as well as the activeingredient itself are capable of releasing methyl isothiocyanate is ofconsiderable importance. Consequently, the byproducts too couldcontribute to the activity in the granules.

The formation of the desired granules my be due to the fact that, on theone hand, byproducts of the structures assumed above are sufficient todisturb the "orderly" crystallization but, on the other hand, suchcompounds have sufficient similarity to the active ingredient itself, sothat they form a disordered conglomerate with the crystals and hence thedesired granules.

The novel process is usually carried out in aqueous solution, either ina one-stage synthesis or in two stages.

In the one-stage reaction, in general carbon disulfite is first added toan aqueous solution of methylamine and diaminoalkylene and an aqueousformaldehyde solution is then added (similarly to the process describedin U.S. Pat. No. 2,838,389, column 6, lines 46-57).

In the reaction in two stages, in general carbon disulfide is firstadded to an aqueous solution of methylamine and diaminoalkylene, thesolution of the resulting carbamate V is then freed from excess carbondisulfide and the solution purified beforehand in this manner is addedto an aqueous formaldehyde solution.

Since the reactions are isothermic but both the intermediate and theactive ingredient are thermally unstable, it is advisable to reduce thereaction temperature by cooling.

In general, the reactions take place at a sufficient rate at above 10°C. At above 50° C., there is already marked formation of undesirabledecomposition products. For this reason, the reactions are usuallycarried out at from 10° to 40° C., preferably from 15° to 30° C.

While the starting materials II and III are reacted with one another asfar as possible in stoichiometric amounts in the one-stage reaction, ingeneral an excess of carbon disulfide (III) is used in the synthesis intwo stages.

Regardless of the reaction, formaldehyde is usually likewise used in aslight excess based on the amount of methylamine (II).

With regard to the use according to the invention, suitablediaminoalkylene compounds of the formula VI are those in which R¹ and R²independently of one another are each hydrogen or C₁ -C₄ -alkyl, such asmethyl, ethyl, propyl, 1-methylethyl, butyl, 1-methylpropyl,2-methylpropyl or 1,1-dimethylethyl, preferably hydrogen, methyl orethyl, in particular hydrogen or methyl, and A is a 1,2-ethylene,1,3-propylene or 1,4-butylene bridge, and these bridges may carry fromone to four C₁ -C₄ -alkyl groups as stated above, preferably one or twomethyl groups.

Preferred diaminoalkylene compounds of the formula VI are1,2-diaminoethylene, 1-(N-methylamino)-2-aminoethylene,1,2-di-(N-methylamino)-ethylene, 1,2-diaminopropylene,1-(N-methylamino)-2-aminopropylene, 1,2-di-(N-methylamino)-propylene,1,3-diaminopropylene, 1-(N-methylamino)-3-aminopropylene,1,3-di-(N-methylamino)-propylene, 1,2-diaminobutylene,1-(N-methylamino)-2-aminobutylene, 1,2-di-(N-methylamino)-butylene,2,3-diaminobutylene, 2-(N-methylamino)-3-aminobutylene,2,3-di-(N-methylamino)butylene, 1,4-diaminobutylene,1-(N-methylamino)-4-aminobutylene and 1,4-(N-methylamino)-butylene.

The use of 1,2-diaminoethylene, 1-(N-methylamino)-2-aminoethylene,1,2-di-(N-methylamino)-ethylene, 1,2-diaminopropylene,1,2-di-(N-methylamino)-propylene and 1-(N-methylamino)-2-aminopropyleneis particularly preferred, and both the pure compounds and mixtures ofthese compounds may be used.

Usually from 0.1 to 10, preferably from 0.2 to 5, in particular from 0.5to 1.5, mol %, based on the amount of methylamine (II) used, ofdiaminoalkylene VI are added to the reaction mixture.

By also adding seed crystals, it is furthermore possible in this processto influence the size of the granules in a conventional manner. Forexample, small granules would be expected in the case of a very highcontent, based on the starting materials, of seed crystals, whereas avery low content of seed crystals would result in larger granules.

Finely divided tetrahydro-3,5-dimethyl-l,3,5-thiadiazine-2-thione (I) isused as seed crystals in an amount from 1.5 to 10, preferably from 2.5to 7.5, in particular from 3 to 6, mol %, based on V, seed crystalshaving a particle size (diameter) of less than 100 μm being employed.Usually, about 90% of the particles should have sizes of from 50 to 5μm. A particle size distribution in which 100% of the particles aresmaller than 100 μm, about 90% are from 50 to 5 μm and about 10% aresmaller than 5 μm is particularly preferred.

In order to achieve a very uniform distribution in the reaction mediumfrom the beginning of the addition of seed crystals, the latter areadded to the reaction medium preferably in the form of an aqueoussuspension.

The suspension of the seed crystals is mixed with the aqueousformaldehyde solution, both in the one-stage reaction and in thereaction in two stages.

The size of the granules obtainable by the novel process can beinfluenced not only by the addition of the seed crystals and the amountof diaminoalkylene VI but also by the rate of addition of the reactants(formaldehyde solution in the one-stage reaction or carbamate solutionin the reaction in two stages), by the intensity of mixing of thereactants during the reaction and by the duration of mixing of thereactants after the end of the addition of formaldehyde solution in theone-stage reaction or of carbamate solution in the reaction in twostages, where these parameters must be determined on the basis ofgeneral technical knowledge, owing to their dependence on the amount ofreactants, the dependence on the geometry of the reaction vessel and thedependence on the method of mixing. The following generally knownrelationships should be noted here: The higher the rate of addition ofthe reactants, the smaller are the granules formed. The more intense themixing of the reactants, the smaller are the granules formed, and inaddition abrasion effects may result in the product having a highcontent of very fine material (fine fraction), which may lead to productdust after drying. The longer mixing is continued after the end of theaddition, the more pronounced are abrasion effects and hence the largeris the fine fraction in the product.

The granules obtainable by the process of the invention are suitable forsoil decontamination in the manner known per se for the activeingredient.

Process Examples EXAMPLE 1

140.5 g of carbon disulfide was added to a mixture of 111 g ofmethylamine, 4.37 g of ethylenediamine, 1.24 g ofN-methylethylenediamine and 520 ml of water while stirring at from 20°to 30° C. After the end of the addition, the reaction mixture wasstirred for 2 hours at 5° C. and then made up to a volume of 800 ml withwater.

The solution thus obtained was then added at from 30° to 50° C. to aprepared mixture of 410 g of a 30% strength formaldehyde solution and400 ml of water.

The resulting tetrahydro-3,5-dimethyl-l,3,5-thiadiazine-2-thionegranules were separated off from the mother liquor, washed and dried.80% of the resulting granules had a diameter of 400 to 500 μm.

EXAMPLE 2

Dithiocarbamate solution was first prepared, by a procedure similar tothat in Example 1, at from 30° to 50° C., from 293.7 g of a 40% strengthmethylamine solution (in water, corresponding to 117.5 g of methylamine)and 2.25 g of ethylenediamine in 300 ml of water by adding 157.8 g ofcarbon disulfide. After unreacted carbon disulfide has been separatedoff, the mixture was made up to a volume of 800 ml with water.

By adding the solution thus obtained to a mixture of 322 g of 40%strength aqueous formaldehyde solution which contains 15 g of seedcrystals (particle size distribution: 100% by weight smaller than 100μm, about 90% by weight from 50 to 5 μm and about 10% by weight smallerthan 5 μm) and 900 ml of water,tetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione granules wereobtained (after isolation, purification and drying) where 100% by weightof the granules had a diameter smaller than 400 μm, about 90% by weightof the particles had a diameter of from 400 to 100 μm and less than 10%by weight had a diameter smaller than 100 μm.

We claim:
 1. A process for the preparation of substantially dust-freegranules of tetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione (I) ofthe formula ##STR4## wherein said granules contain dimers or higherpolymers of the active ingredient, in which two nitrogen atoms of thedimers or higher polymers not belonging to the same molecule I arelinked to one another by a 1,2-ethylene, 1,3-propylene or 1,4-butylenebridge, which process comprises: reacting methylamine (II) and from 0.1to 10 mol %, based on (II) of at least one diaminoalkylene of theformula VI

    R.sup.1 -NH-A-NH-R.sup.2                                   VI

where R¹ and R² independently of one another are each hydrogen or C₁ -C₄-alkyl and A is a 1,2-ethylene, 1,3-propylene or 1,4-butylene bridge,and these bridges may carry from one to four C₁ -C₄ -alkyl groups, withcarbon disulfide (III) and formaldehyde (IV).
 2. A process as defined inclaim 1, wherein in a first stage methylamine (II) and of from 0.1 to 10mol %, based on (II) of at least one diaminoalkylene (VI) are reactedwith carbon disulfide (II) to form the corresponding ammonium salts ofthe dithiocarbamic acid derivatives based on II and VI and the resultingammonium salts of the dithiocarbamic acid derivatives are reacted withformaldehyde.
 3. A process as defined in claim 1, wherein the reactionis carried out in the presence of finely dividedtetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione (I).
 4. A process asdefined in claim 1, wherein in the diaminoalkylene of the formula VI, R¹or R² independently of one another are hydrogen, methyl or ethyl and Ais an ethylene bridge which in turn may carry one to two methyl or ethylgroups.
 5. A process as defined in claim 3, wherein the finely dividedtetrahydro-3,5-dimethyl-1, 3,5-thiadiazine-2-thione (I) is used in anamount from 1.5 to 10 mol %, based on the methylammonium salt ofN-methyldithiocarbamic acid (V).
 6. A process as defined in claim 3,wherein the finely dividedtetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione (I) has a particlesize of less than 100 μm.
 7. A process as defined in claim 3, whereinthe finely divided tetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione(I) is used in the form of an aqueous suspension.
 8. Substantiallydust-free granules oftetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione, obtained by theprocess of claim 7, wherein said granules contain dimers or higherpolymers of the active ingredient, in which dimers or higher polymerstwo nitrogen atoms not belonging to the same molecule I are linked toone another by a 1,2-ethylene, 1,3-propylene or 1,4-butylene bridgecorresponding to substituent A.
 9. A method of soil decontamination,which comprises treating the soil with an effective amount ofsubstantially dust-free granules oftetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione as defined in claim8.
 10. A method of controlling nematodes, germinating plants and soilfungi, which comprises treating the earth with an effective amount ofsubstantially dust-free granules oftetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione as defined in claim8.